Mexico's drug violence that has killed 23,000 people could rage for years before being curtailed, judging by past anti-crime fights in Italy, Colombia and the United States, the government said on Tuesday.

Security Minister Genero Garcia Luna told the Reuters Latin American Investment Summit in Mexico City that his country could not expect a quick victory in the army-backed offensive launched in late 2006 against powerful drug cartels fighting over lucrative smuggling routes into the United States.

"In every case, the average is between six and eight years," Garcia Luna said in his headquarters in the Mexican capital. "In the case of Italy, in the case of Colombia, New York and Chicago, the curve was six years on average ... In Italy it lasted almost seven years."

"I hope that in Mexico it will be sooner," he added.

With strong support from Washington, President Felipe Calderon has sent more than 70,000 soldiers, elite navy units and federal police across Mexico to take on the cartels.

Despite record drug seizures and arrests, violence has escalated to horrifying levels. Once-quiet manufacturing and colonial tourist towns feel terrorized by daylight shootouts between rival gangs and the army.

Some companies are freezing investment along the U.S. border, local business leaders say.

'WORKING FORCEFULLY'

Garcia Luna said the government is doing all it can to protect the economy from any drug war impact.

"Mexico is working forcefully and institutionally so people keep investing in Mexico," he said.

He said a decision to switch control of security operations from the army to federal police in Ciudad Juarez was bearing fruit despite a surge in violence in April, with killings spiking to 20 a day in the city across from El Paso, Texas.

"We've had federal police (in control) in Ciudad Juarez for almost 20 days ... and we are beginning to see signs of improvement in public security," Garcia Luna said.

In a new challenge, Garcia Luna said the Zetas gang, the former armed wing of the powerful Gulf cartel across from Texas, is now a cartel in its own right, with links to South America cocaine suppliers.

The rift between the Zetas and their former employers has set off a burst of killings in Mexican towns near the Laredo-Brownsville area of Texas.

Severed heads and bodies hung from bridges are becoming commonplace from Mexico's Caribbean to its northern border with the United States. A spate of civilian killings, including infants caught up in shootouts between troops and hitmen, has hurt support for Calderon's drug war.

The killing of three people linked to the U.S. consulate in Ciudad Juarez in March provoked outrage from President Barack Obama, whose administration is providing Mexico with millions of dollars in anti-drug war equipment and police training.

Garcia Luna said a major priority was still the cleaning up of Mexico's notoriously corrupt and poorly paid police forces that often work with drug gangs.

He aims to eliminate Mexico's 2,000 municipal forces that are ill-equipped to deal with heavily armed drug hitmen and create a unified force based on state-level police.